Efficient valley polarization of charged excitons and resident carriers in Molybdenum disulfide monolayers by optical pumping

Abstract

The roadmap of future innovative device developments foresees the reduction of material dimensions down to nanometer scale and the incorporation of novel degrees of freedom. For instance, electrons and holes in 2D semiconductors like MoS2 monolayers exhibit a unique coupling between the spin and the crystal momentum, also referred to as the valley. A crucial requirement for future applications is therefore the possibility to initialise the spin/valley degree of freedom in these materials. Here we investigate the optical initialisation of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report in photoluminescence a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation. We reveal efficient valley initialisation of positively-charged excitons, which have so far proved to be elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged excitons present a polarization of 70% which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes––as recently shown in tungsten-based monolayers.